1. Field of the Invention
The present invention relates to a focus detecting system and, more particularly, to a focus detecting system which detects a focusing state of an optical system which is set to cause change in the image formation state of an image of an object in accordance with data processing thereof, the image being formed on a time-serial signal output type photoelectric transducer element such as an image pickup tube or a solid image-pickup element such as a CCD, BBD or CID.
2. Description of the Prior Art
Various conventional focus detecting systems for optical systems have been proposed. An increasing number of focus detecting systems have been proposed which utilize solid image-pickup elements such as the CCD, BBD or CID, the practicability of which has recently become notable. The characteristic feature of the image pickup tubes and solid image-pickup elements as described above resides in that extremely small photoelectric transducer elements are incorporated which produce time-serial electric signals of small picture elements of an image formed on an image formation plane. Therefore, the image pickup tube and the solid image-pickup element allow subsequent time-serial processing of the signals and are therefore suitable for processing with electric circuitry, in contrast to conventional systems which have a plurality of usual photoelectric elements on which the image of an object is formed to produce photoelectric conversion signals of the image. Since the photoelectric transducer elements constituting the solid image-pickup means, unlike the usual photoelectric transducer elements, serve to accumulate over a given period of time charge obtained by photoelectrically converting the energy of light incident thereon and to output time-serial signals, the area for each of these elements may be made very small so that the obtained time-serial signal train provides image signals of good resolution.
An example of a focus detecting system is disclosed in U.S. Pat. Nos. 3,364,815, 3,621,136 and 3,896,304 (German DAS No. 23 49 311) wherein a time-serial output type photoelectric transducer element of the type described above is used as an image sensing means, and the image formation state of an image of an object formed by an optical system is detected through the time-serial output signal from the image sensing means to thereby detect the focusing state of the optical system on the object. The focus detecting systems disclosed in these U.S. Patents have a common feature in that the time-serial output signal from the time-serial output type image sensing means (an image pickup tube or a solid image-pickup element) is supplied to a differentiating circuit, that is, a kind of high-pass filter to obtain a differential signal representing the change in the image signal, and the detection of the image formation state of the image is performed according to this differential signal.
However, when a high-pass filter is used to obtain a change in the image signal, the image signal abruptly rises at the initial moment of light reception by the photoelectric transducer element or immediately thereafter. Therefore, the change in the image signal is detected as magnified by the high-pass filter, making it difficult to detect the actual change in the image signal. For example, the high-pass filter produces abruptly high outputs and subsequent oscillating outputs for a considerable length of time. This significantly degrades the focus detecting precision.